In certain industries, such as the aerospace industry, tolerances in fastener joints are critical because of the high loads carried by the joint and because the fatigue life of the joint is somewhat dependent on these tolerances. The production equipment actually available in these industries, however, makes these tolerances very difficult, if not impossible, to obtain. While production facilities for fasteners are able to obtain relatively good tolerance ranges, the same has not been true of the hole making operation. This is because the hole forming tool such s a drill or reamer may flex or bend as within the holes to make a discrepancy and because the workman may unintentionally vary the angle of the tool with respect to the desired centerline that the formed hole is to have. The most prevalent discrepancy produced by the above mentioned problems is that part of the hole is oval rather than circular as desired especially on that side of the work pieces from which the work is being performed. This irregularity is generally referred to in the trade as "bell mouth".
These discrepancies have generally caused fretting that reduces the fatigue life of the joint. Also, these discrepancies have made it difficult to seal the joint against introduction of a corrosive atmosphere into the joint while in use in the field.